The present invention relates to a catalytic converter which is suitably applicable to purification of an exhaust gas of an automobile, or the like.
Recently, a metallic honeycomb structure has been attracted a good deal of public attention besides a porous ceramic honeycomb structure as a carrier, or the like, for a catalyst for purifying pollutants such as nitrogen oxide (NO.sub.x), carbon monoxide (CO), hydrocarbon (HC), and the like, contained in an exhaust gas discharged from an internal combustion engine of an automobile, or the like.
Additionally, there has been earnestly desired development of a heater, or the like, which can reduce a discharge of pollutants upon cold start as regulations on an exhaust gas are tightened.
The present applicant previously proposed a honeycomb heater in which a honeycomb structure is provided with a resistor adjusting mechanism (U.S. Pat. No. 5,063,029). The present applicant further proposed a method for holding a honeycomb heater by covering a circumference of the honeycomb heater with a metallic band by means of an insulating substance such as a ceramic mat, cloth, or the like (U.S. Pat. No. 5,202,548).
The aforementioned method discloses a method for protecting a resister adjusting mechanism of a heater by an insulation. However, the heater disclosed in U.S. Pat. No. 5,063,029 has a possibility that an inorganic adhesive drops off under a severe driving conditions (particularly, vibrations and thermal shocks) of an automobile. The heater disclosed in U.S. Pat. No. 5,202,548 has a possibility that a heater is deformed by horizontal and vertical vibrations, resulting in breakage of a spacer or wear of an insulating mat.
The present applicant has further studied so as to develop a new heater unit which is free from breakage and exfoliation of a honeycomb heater against an expansion or a shrinkage caused by vibrations or thermal shocks under a severe condition of an automobile. The new heater unit was disclosed in SAE Technical Paper Series 940466. The heater unit is very preferable because it hardly has deformation or breakage of a honeycomb heater.
The heater unit has a structure that a honeycomb heater is hold by a housing by means of a metallic, flexible holding member. SAE Technical Paper Series 940466 also discloses a flexible electrode structure, a ring which limits a gas flow, and a disposition of a light-off catalyst downstream of the honeycomb heater.
An embodiment of a constitution of a conventional catalytic converter is hereinbelow described with reference to FIG. 19. Housings 12 and 14 fixedly install a honeycomb heater 20 and a catalytic converter 30. Specifically, a honeycomb heater is fixed to a housing 12 by means of a holding member (not shown) so as to form a heater unit. Similarly, a catalytic element 30 is fixed to a housing 14 by means of a holding member (not shown) so as to form a catalytic unit. When a catalytic converter is produced, a duct 16, the heater unit, and the catalyst unit are fixed.
A numeral number of througholes are formed in parallel in a fluid-flow direction in a honeycomb structure 22 constituting a honeycomb heater 20 and a honeycomb structure 32 constituting a catalytic element 30. A fluid such as an exhaust gas, or the like, of an internal combustion engine is introduced to an inflow end surface 22s of the honeycomb structure 22 of the honeycomb heater 20 from a duct 16. The fluid, then, passes through the throughholes and flows out of an outflow end surface 22t. An electrode 28 supplies electricity to a honeycomb structure 22, and the fluid is heated upon passing through the throughholes of the honeycomb structure 22.
The fluid flows out of the outflow end surface 22t of the honeycomb structure 22, passes through a gap G, is introduced to an inflow end surface 32s of a honeycomb structure 32 of a catalytic element 30, passes through throughholes, and flow out of an outflow end surface 32t. Since a surface of a partition wall for forming the throughholes in the honeycomb structure 32 is covered with a catalytic composition, active components such as noble metals contained in the catalytic composition remove pollutants such as oxygen nitride, carbon monoxide, hydrocarbon, or the like, in an exhaust gas by oxidation or reduction.
When an exhaust gas has not been warmed, for example, when an engine of an automobile is started up, the catalytic composition of the catalytic element 30 is not activated, and therefore, the pollutants in the exhaust gas cannot be removed. Accordingly, the honeycomb heater 20 heats the exhaust gas up to a light-off temperature or more so as to activate the catalytic composition and improve an efficiency in removing the pollutants in the exhaust gas.
In a conventional catalytic converter, a diameter D.sub.2 of an inflow end surface 32s of a honeycomb structure 32 was almost the same as a diameter D.sub.1 of the outflow end surface 22t of a honeycomb structure 22. A honeycomb structure 32 constituting a catalytic element 30 is required to have a certain degree of volume to obtain a predetermined exhaust-gas purifying ability by ensuring a surface area for loading a catalytic composition thereon and to obtain an O.sub.2 storage (keeping) ability for detecting deterioration of a catalyst.
On the other hand, a catalytic converter is also required not to deteriorate properties of the internal combustion engine and to lower a pressure loss as much as possible caused when the fluid passes through the honeycomb structure 32.
In order to satisfy these requirement, it is preferable to make the diameter D.sub.2 of the honeycomb structure 32 constituting the catalytic element 30 as large as possible, and to make a length L.sub.2 of the honeycomb structure 22 in the axial direction as short as possible. In that case, the diameter D.sub.1 of the honeycomb structure 22 of the honeycomb heater 20 also becomes large. Therefore, as a result there has been a problem that a vibration resistance of the honeycomb structure 22 is lowered, and a risk of breakage of the honeycomb structure 22 in the worst case by vibrations or the like upon driving with a high burden.
When an alternator attached to the internal combustion engine is used as a source for supplying electricity to the honeycomb heater 20, the honeycomb heater 20 is required to have a high resistance of usually 200 m.omega. or more because of properties of the output voltage. In this case, in order to obtain a high resistance, the length L.sub.1 of the honeycomb structure 22 is small and the number of slits arranged in the honeycomb structure 22 is high, and thus the aforementioned vibration resistance is highly concerned about.
When the diameter D.sub.1 is made small in view of vibration resistance of the honeycomb structure 22, the diameter D.sub.2 of the honeycomb structure 32 becomes small. In this case, there are caused problems that a pressure loss increases and output properties of an internal combustion engine deteriorate.
On the other hand, the conventional converter is aimed to rapidly heat the honeycomb structure 32 by a thermal energy obtained by heating a fluid flowing through the honeycomb structure 22 by electrically heating the honeycomb structure 22. Accordingly, a gap G between the honeycomb structures 22 and 32 was formed as small as possible. In this case, a region where a fluid such as an exhaust gas is present in a circumference of the honeycomb heater 20 and a region where exothermic reaction is not caused electrically also in a circumference of a honeycomb structure 22. Accordingly, there has been a problem that a fluid flowing through a circumferential portion 33 of the honeycomb structure 32 is not heated after an engine is started up particularly in a cold season, thereby impeding the exhibition of a purification ability in this range.
As a volume of the honeycomb structure 22 becomes large, a heat capacity of the honeycomb structure 22 becomes large and temperature-rising speed of the honeycomb heater 20 slows down. However, since a honeycomb heater is desired to have a high temperature rapidly, a volume of a honeycomb structure 22 is desirably small.
Further, because of a convenience of designing an exhaust system of an automobile, a diameter of the duct is small, and generally, an area of the inflow end surface 22s of the honeycomb structure 22 is larger than that of the outflow end surface 16s of a duct 16. If so, a fluid flows into a portion of the honeycomb structure 22, and the portion is prone to deform by a heat. This is particularly remarkable when an automobile runs at high speed and caused by both rises in speed and temperature of an exhaust gas. Hence, an area of the inflow end surface 22s of the honeycomb structure 22 is desirably a little larger than that of the outflow end surface 16s of the duct 16.
However, if a volume and an area of a horizontal cross-section of the honeycomb structure 22 are made small, a problem of pressure loss in the honeycomb structure 32 as described above. Therefore, it was difficult to miniaturize the honeycomb structure 22.
Additionally, in a conventional catalytic converter, a gap G between an outflow end surface 22t of a honeycomb structure 22 and an inflow end surface 32s of a honeycomb structure 32 was made small so as to prevent a fluid such as an exhaust gas from being cooled down, i.e., to avoid thermal loss after the fluid flows out of the outflow end surface 22t of the honeycomb structure 22 and before the fluid flows in through the inflow surface 32s of the honeycomb structure 32. For example, the present applicant disclosed in U.S. patent application Ser. No. 08/412,279, now U.S. Pat. No. 5,614,155 a honeycomb structure 22 having a length L.sub.1 in the axial direction of 5-20 mm. The honeycomb structure 22 preferably has a volume of 30-150 cm.sup.3. When such a honeycomb structure 22 is used, generally, the gap G was determined to be 10 mm or less, and typically 5-8 mm. Incidentally, in U.S. patent application Ser. No. 08/412,279 a distance between the inflow end surface 22s of a honeycomb structure 22 and the outflow end surface 16s of a duct 16 is preferably 3 mm or less.